1. Field of the Invention
This invention relates to a process for chemically maintaining by electrodialysis an electroless copper plating solution and, more particularly, to an improved method of and apparatus for measuring and controlling the net hydroxyl ion (OH.sup.-) production that is put into and left in the electroless copper plating bath during the electrodialysis process.
2. Description of the Prior Art
An electroless copper plating solution contains copper, usually in the form of copper sulfate, a reducing agent such as formaldehyde, a chelating agent, and an alkali metal hydroxide as essential components.
In the continued use of an electroless copper plating bath, copper sulfate, formaldehyde, and sodium hydroxide are consumed. Depletion of these components create a need for replenishing them. Additionally, as the electroless copper plating bath is used, by-product components are produced that have an inhibiting effect upon the chemical plating action and accumulate in the plating solution. Most notably, these by-product components are sodium sulfate and sodium formate.
As disclosed in U.S. Pat. No. 4,289,597 issued on Sept. 15, 1981 to David W. Grenda, and in the copending application for U.S. patent bearing Ser. No. 691,095 filed on Jan. 14, 1985 by Emmanuel Korngold now U.S. Pat. No. 4,600,493, the disclosures of which patent and copending application for patent, by reference, are incorporated herein, such by-product components may be removed, by electrodialysis from an operating electroless copper plating bath while replacing them with freshly generated hydroxide. Electrodialysis is a form of dialysis in which an electric current is used to aid the separation of substances that ionize in solution by providing a driving electric potential to cause the transference of ions across semi-permeable membranes.
During the normal operation of an electroless copper plating bath, a chemical reaction, as follows, takes place: EQU Cu SO.sub.4 +4 NaOH+2 HCHO.fwdarw.Cu.degree.+Na.sub.2 SO.sub.4 +2 NaOOCH+H.sub.2 +2 H.sub.2 O
For every four moles of sodium hydroxide (NaOH) consumed, two moles of sodium formate (NaOOCH) and one mole of sodium sulfate (Na.sub.2 SO.sub.4) are produced. Consequently, for each complete replacement of all of the copper in the plating bath, termed a "cycle," a certain amount of sulfate and formate is produced in the bath.
With continued use and replenishment, the sulfate and formate concentrations, increase steadily until the concentrations reach a level where the loss due to volume growth disposal and production rates are balanced. This is a so-called "steady state" condition. During the time between the preparation of a fresh bath and its steady state condition, the bath may display a gradual change in its performance characteristics. Thus, a "cycled" bath is usually always less stable against autocatalytic decomposition than a fresh bath. This is due primarily to the accumulation of sulfate and formate anions.
In traditional electrodialysis, very small electrical currents are used since charged ions only are being separated. The version of electrodialysis with which the present invention is concerned is significantly different since large electrical currents are needed. Most of this current is used to generate hydroxyl ions and also to transport them across the semi-permeable membranes.
In this version of the electrodialysis process, water in the catholyte of an electrodialytic cell is electrolyzed to form hydroxyl anions at the cathode. These electrosynthesized hydroxyl anions subsequently migrate across an anion permeable membrane into an electroless copper plating bath solution which is contained in an intermediate compartment between two such anion permeable membranes. At the same time sulfate and formate anions, together with some hydroxyl anions, transfer across the second membrane into an anolyte solution in the anode compartment of the cell.
As a result of this process, three stoichiometric exchanges take place, as follows: EQU 2 OH.sup.- for 1 SO.sub.4 = (1) EQU 2 OH.sup.- for 2 OOCH.sup.- ( 2) EQU 1 OH.sup.- for 1 OH.sup.- ( 3)
Hence, the overall net exchange is: EQU 4 OH.sup.- for 1 SO.sub.4 = plus 2 OOCH.sup.- ( 4)
Thus, for every mole of sulfate and two moles of formates removed, four moles of hydroxides are introduced. This is a perfect reversal of the reaction which takes place during electroless copper plating where four moles of hydroxides are consumed, producing one mole of sulfate and two moles of formates.
As the concentration of the sulfate and formate decreases, a correspondingly greater percentage of OH.sup.- will be transported across the membrane. Thus the net rate of OH.sup.- change will approach zero. As a result, the electroless copper plating bath cannot be overreplenished in caustic.
An important aspect of the electrodialysis process for the chemical maintenance of an electroless copper plating bath thus is the net hydroxyl ion production, specifically the amount of hydroxide or OH.sup.- actually put into and left in the electroless copper solution.
It has been proposed in the prior art to measure the net hydroxide production by titration of the electroless copper solution as a function of time of operation of the electrodialysis apparatus. A proposal has also been made to make this measurement by titrating aliquots of electroless copper solution taken at the entry and at the exit of the electrodialysis apparatus to get instantaneous net OH.sup.- production values. It has also been proposed to use two hydrogen ion, or "pH" probes or sensors, one at the entrance to and one at the exit from the electroless copper solution compartment of the electrodialysis apparatus, to measure the hydrogen ion differential and thus the instantaneous change in hydroxide. The latter method appears to be the only practical prior art method lending itself to automatic operation, yet it suffers from many disadvantages. These disadvantages stem from the fact that the electroless copper solution during plating conditons is normally both hot and caustic with a high pH level. Hot caustic is a difficult environment for pH probes. So applied, the useful life of the pH probe would be severely curtailed. Additionally, measurement of a small increase in OH.sup.- at a high pH level is difficult. As a result, such measurements would be inaccurate and unreliable.
Thus, there is a need and a demand for an improved method of and apparatus for measuring the amount of OH.sup.- that is actually put into and left in the electroless copper plating solution during the electrodialysis and electrosynthesis process. There is a need and a demand also for an improved method of and apparatus for effecting an appropriate control action in response to such measurement for automatically adjusting the amount of OH.sup.- that is put into and left in the electroless copper plating solution in accordance with the requirements of the electroless copper plating bath during the operation thereof.